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[Qemu-discuss] Incremental drive-backup with dirty bitmaps

From: Bharadwaj Rayala
Subject: [Qemu-discuss] Incremental drive-backup with dirty bitmaps
Date: Wed, 23 Jan 2019 00:59:27 +0530


TL(Cant)R: I am trying to figure out a workflow for doing incremental
drive-backups using dirty-bitmaps. Feels qemu lacks some essential features
to achieve it.

I am trying to build a backup workflow(program) using drive-backup along
with dirty bitmaps to take backups of kvm vms. EIther pull/push model works
for me. Since drive-backup push model is already implemented, I am
going forward with it. I am not able to figure out a few details and
couldn't find any documentation around it. Any help would be appreciated

Context: I would like to take recoverable, consistent, incremental
backups of kvm vms, whose disks are backed either by qcow2 or raw images.
Lets say there is a vm:vm1 with drive1 backed by image chain( A <-- B ).
This are the rough steps i would like to do.

Method 1:
1. Perform a full backup using `drive-backup(drive1, sync=full, dest =
/nfs/vm1/drive1)`. Use transaction to do `block-dirty-bitmap-add(drive1,
bitmap1)`. Store the vm config seperately
2. Perform an incremental backup using `drive-backup(drive1,
sync=incremental, mode=existing, bitmap=bitmap1, dest=/nfs/vm1/drive1)`.
Store the vm config seperately
3. Rinse and repeat.
Recovery(Just the latest backup, incremental not required):
    Copy the full qcow2 from nfs to host storage. Spawn a new vm with the
same vm config.
Temporary quick recovery:
    Create a new qcow2 layer on top of existing /nfs/vm1/drive1 on the nfs
storage itself. Spawn a new vm with disk on nfs storage itself.
Issues i face:
1. Does the drive-backup stall for the whole time the block job is in
progress. This is a strict no for me. I didnot find any documentation
regarding it but a powerpoint presentation(from kaskyapc) mentioning it.
(Assuming yes!)
2. Is the backup consistent? Are the drive file-systems quiesced on backup?
(Assuming no!)

To achieve both of the above, one hack i could think of was to take a
snapshot and read from the snapshot.

Method 2:
1. Perform a full backup using `drive-backup(drive1, sync=full, dest =
/nfs/vm1/drive1)`. Use transaction to do `block-dirty-bitmap-add(drive1,
bitmap1)`. Store the vm config seperately
2. Perform the incremental backup by
     a. add bitmap2 to drive1 `block-dirty-bitmap-add(drive1, bitmap2)`.
     b. Take a vm snapshot with drive1(exclude memory, quiesce). The drive1
image chain is now A<--B<--C.
     c. Take incremental using bitmap1 but using data from node B.
`drive-backup(*#nodeB*, sync=incremental, mode=existing, bitmap=bitmap1,
     d. Delete bitmap1 `block-dirty-bitmap-delete(drive1, bitmap1)`
     e. Delete vm snapshot on drive1. The drive1 image chain is now A <--B.
     f. bitmap2 now tracks the changes from incrementa 1 to incremental 2.

Drawbacks with this method would be(had it worked) that incremental backups
would contain dirty blocks that are a superset of the actual blocks that
are changed between the snapshot and the last snapshot.(Incremental x would
contain blocks that have changed when incremental x-1 backup was in
progress). But there are no correctness issues.

*I cannot do this because drive-backup doesnot allow bitmap and node that
the bitmap is attached to, to be different. :( *
Some other issues i was facing that i worked around:
1. Lets say i have to backup a vm with 2 disks(both at a fixed point in
time, either both fail or both pass). To atomically do a bitmap-add and
drive-backup(sync=full) i can use transcations. To achieve a backup at a
fixed point in time, i can use transaction with multiple drive-backups. To
either fail the whole backup or succeed(when multiple drives are present),
i can use completion-mode = grouped. But then i cant combine them as its
not supported. i.e, do a
    Transaction{drive-backup(drive1), dirty-bitmap-add(drive1,
bitmap1),drive-backup(drive2), dirty-bitmap-add(drive2, bitmap1),
 Workaround: Create bitmaps first, then take full. Effect: Incrementals
would be a small superset of actual changed blocks.
2. Why do I need to dismiss old jobs to start a new job on node. I want to
retain the block-job end state for a day before i clear them. So i set
auto-dismiss to false. This doesnot allow new jobs to run unless the old
job is dismissed even if state=concluded.
 Workaround: no workaround, store the end-job-status somewhere else.
3. Is there a way pre 2.12 to achieve auto-finalise = false in a
transaction. Can I somehow add a dummy block job, that will only finish
when i want to finalise the actual 2 disks block jobs? My backup workflow
needs to run on env's pre 2.12.
 Workaround: Couldnot achieve this. So if an incremental fails after block
jobs succeed before i can ensure success(have to do some metadata
operations on my side), i retry with sync=full mode.

*So what is the recommeded way of taking backups with incremental bitmaps
? *
Thanks you for taking time to read through this.


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